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Review of the literature suggested possible deficiencies of micronutrients in soils of North Western Zambia. Soil analysis, pot and field experiments were employed to investigate possible deficiencies. The pot experiments investigated how raising soil pH through liming influenced extractable micronutrients and their uptake by plants. Plant Mo and Ca were positively correlated with soil pH, while Mn and Zn were inversely correlated, aggravating the zinc inadequacy on all soils and that for Mn on arenosols. Effects of liming on plant uptakes of micronutrients generally followed the same trends as those on soil extraction. Incubating the soil under grass house conditions was found to influence amounts of extractable micronutrients, increasing most times above their levels before the soil was incubated. Field experiments generally showed that applying micronutrients were beneficial to crop yield only at some sites. Grain yield variables responded variously and were most significantly correlated with overall grain yield. Soil analysis usefully predicted deficiencies of Zn for both maize and soybean. However, predictions for B and Mo were ideal for soybean than maize. Cu also seemed to have been wrongly predicted for soybean. However, plant nutrient concentration was better at predicting nutrient status in relation to grain yield, but the lower limits of the suggested optimal concentration ranges may need to be worked out again. Soybean was found to have more micronutrient latent deficiencies at majority of the sites than maize. One of the characteristics of applied micronutrients was their beneficial residual effects of crop yield. The residual benefit was also noticed on maize when the fertilisers were directly applied to soybeans a season before, suggesting a possibility of crop rotation, thus spreading the costs. Results would suggest changing the current fertiliser recommendations in the region.

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Doctor of Philosophy / Department of Horticulture and Natural Resources / Sunghun Park / Drought and heat stress are two of the biggest constraints to global food production. Abiotic stress response pathways are complex and consist of osmotic adjustors, macromolecule stabilizers, and antioxidants to counteract the damaging nature of abiotic stress induced reactive oxygen species (ROS) accumulation. In this work, we studied the effect of overexpression of an Arabidopsis glutaredoxin, AtGRXS17, on heat tolerance in maize (Zea mays L.) and drought tolerance in rice (Orzya sativa L.). Glutaredoxins (GRXs) are proteins cable of reducing disulfide bonds, therefore regulating the cellular redox status, and require glutathione for regeneration. Ectopic expression of AtGRXS17 in maize resulted in increased heat stress tolerance during flowering. AtGRXS17 enhanced heat tolerance by increasing kernel set and total grain yield during heat treatments, compared to wild type controls. Our results indicated that AtGRXS17-expressing maize plants produce heat tolerant pollen with higher germination rates than wild type when challenged during heat treatments. Furthermore, AtGRXS17-expressing plants were less susceptible to post pollination heat induced kernel abortion. Rice plants expressing AtGRXS17 were also tolerant to abiotic stress. AtGRXS17-expressing rice was more tolerant to drought stress challenges and consistently survived drought treatments. A nontargeted metabolomics study revealed distinct changes in profiles of key metabolite groups in response to drought stress. Soluble sugars and amino acids accumulate as osmotic adjustors while antioxidants, such as glutathione, accumulate to mediate ROS accumulation and regulate redox activity. All genotypes accumulated amino acids, soluble sugars, and raffinose family oligosaccharides in response to drought stress. Our results indicated AtGRXS17-expression affected several pathways known to increase drought tolerance. Altered sugar metabolites suggested a redox modulation of sucrose synthase activity and significant increases in the secondary sulfur assimilation pathway metabolites suggested altered sulfur metabolism. This research provides new insights into ability of GRXs to improve heat tolerance and crop yield in maize and functions of GRXs in affecting metabolite profiles contributing to increased drought tolerance in rice.

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The ongoing quest for a better understanding of adoption behaviour, and more specifically the search for relevant, and meaningful behaviour determinants that can be useful in the understanding, analysis and change of adoption behaviour, has prompted this study. It was specifically focused on the role of intervening variables and their influence relative to the commonly used independent variables. A pre-tested, structured questionnaire was used to collect data from 113 farmers randomly selected to represent five percent samples of four villages selected to represent the biggest variation in terms of climatic conditions within the Njombe district of Tanzania. Correlations, chi-square, and regressions were used to determine the relationship between the independent and the dependent variables. The results show that most of the farmers’ (97.3 percent) production efficiency falls well below the optimum maize yield of about 40 bags per acre. Various independent and intervening factors were found to influence adoption. In general, the intervening variables show, without exception, much stronger influence relationships with adoption behaviour than is the case with independent variables. Also, unlike what is a common phenomenon among independent variables, these relationships show great consistency, which further supports the research hypothesis. The most convincing evidence in support of the critical role of intervening variables in decision making and adoption behaviour are the regressions, which explain about 73.2 to 93.6 percent of the variation in adoption as compared to the mere 6.0 to 32.9 percent of the independent variables. The explanation for this highly significant difference is that the intervening variables are probably the immediate and direct determinants of adoption behaviour and that the influence of intervening variables only becomes manifested in adoption behaviour via the intervening variables. This explains why the influence of independent variables is much smaller and more inconsistent than that of the intervening variables. The practical implications of these findings are that the emphasis in the analysis and understanding of adoption behaviour should be on the intervening variables. They lend themselves as so-called “forces of change” and thus represent the focus of extension endeavours, but also as criteria for evaluation and monitoring. From the study arise various issues that call for further research like refinement of the measurements. / Thesis (PhD(Agrarian Extension))--University of Pretoria, 2008. / Agricultural Economics, Extension and Rural Development / PhD / unrestricted

An efficient approach to combat the accumulation of aflatoxin is the development of germplasm resistant to infection and spread of A. flavus in maize, one of the most important cereal grains in the world. Lipoxygenases (LOXs) are a group enzymes that catalyze oxygenation of polyunsaturated fatty acids (PUFAs). LOX derived oxilipins play critical roles in plant defense against pathogens such as A. flavus. The objectives of this study were to report sequence diversity and expression patterns for all LOX genes, and map their effect on aflatoxin accumulation via linkage and association mapping. Genes GRMZM2G102760 (ZmLOX 5) and GRMZM2G104843 (ZmLOX 8) fell under previously published QTL in one of four mapping populations and appear to have a measurable effect on the reduction of aflatoxin in maize grains. The association mapping result shows 19 of the total 215 SNPs found within the sequence of the ZmLOXs were associated with reduced aflatoxin levels.

The study revealed that the subsidy programs implemented in Malawi over the years have exposed many farmers to the advantages of using improved farm inputs. Almost 86 percent of the participants had a chance to grow hybrids in their farming career and out of that number 89 percent continues to grow them on a yearly basis and 76 percent indicated that hybrids have turned out to be their preferred variety. It has also revealed that complete removal of subsidy will result in the demand for inputs from subsidy eligible farmers to be removed from the market. This is based on the fact that only 5 percent of the participants were willing to pay for the improved input packages at market prevailing price.

Understanding the role of phytohormone auxin in defense responses is one of the vital tools for plant breeders to develop maize germplasm lines that exhibit high resistance to Aspergillus flavus and subsequent aflatoxin accumulation. Besides its critical role in different developmental processes throughout the life cycle of plants, auxin is also involved in the network of plant-pathogen interaction as demonstrated in previous studies. However, the actual mechanism for the auxin signaling pathway leading to resistance is unknown. Therefore, the critical gap in the knowledge base is a lack of understanding of the role of auxin signaling in pathogen resistance in maize. Continuation of this gap is an important problem because fungal resistance is a highly quantitative trait and breeding for resistance is a challenge. A complete understanding of the auxin mechanism in resistance could lead the production of corn hybrids with resistance to A. flavus and aflatoxin accumulation. The focus of this research was to determine the effect of exogenous auxin on A. flavus growth and production of aflatoxin in growth media. In addition, auxin levels, the amount of aflatoxin, and fungal growth in three resistant (Mp313E, Mp715, and Mp719) and one susceptible (B73) germplasm line were determined. As a result, auxin significantly increased mycelium growth and significantly decreased aflatoxin at a high concentration in potato dextrose broth under the lab conditions. Under the field conditions, auxin levels were low in resistant lines but did not change in response to A. flavus infection. Susceptible line had high auxin levels and auxin levels significantly decreased in response to A. flavus infection.

Iron is an essential micronutrient that plays a role in essential processes in all living organisms. Because iron deficiency anemia is the number one human nutritional deficiency worldwide, research has been focused on studying biofortification, a method of plant breeding focused on increasing the nutrient content of the grain. In order to effectively implement this strategy, research to understand the molecular mechanisms surrounding iron uptake and maintenance within plants is necessary. A major goal of this work lies in exploring iron signaling in Arabidopsis thaliana and further characterization of the yellowstripe1-like1yellowstripe1-like3 (ysl1ysl3) double mutant plant. As shown here, iron signaling in arabidopsis appears to be regulated by a long distance signal derived in the shoots. The ysl1ysl3 double mutant is a plant shown to have severe mis-regulation of iron regulated genes. Here, I show ysl1ysl3 mutant plants are unable to respond to iron deficiency properly and lose signaling ability. It has been proposed that these defects are due to iron accumulation in the shoots but that appears untrue suggesting alternative functions for YSL1 and YSL3 in iron signaling. Another goal of the work included here seeks to elucidate alternative roles for ZmYS1 in maize outside of primary acquisition of iron from the soil. Here, ZmYS1 expression is shown via promoter::GUS analysis expressing throughout the shoot in many cell-types consistent with a role in the movement of iron between leaf tissues. Interestingly, expression in germinating seeds suggests ZmYS1 may also play a role in remobilization of stored iron in developing embryos.